1. Field of the Invention
This invention relates to a facsimile communication method and in particular to a digital facsimile communication method capable of preventing the occurrence of false operation induced by echo.
2. Description of the Prior Art
As shown in FIG. 1a, in the case of conducting a facsimile communication through the public telephone lines, a calling station A first places a call by dialing as indicated by the dotted line. In response thereto, a called station B returns a CED (Called Station Identification) signal and then a DIS (Digital Identification) signal to the calling station A. In the case where the calling station A is to function as a transmitting station, the calling station A transmits a DCS (Digital Commond) signal and then a phase-adjustment and training signal to the called station B, which, then, returns a CFR (Confirmation to Receive) signal to the calling station A as a response. Then, the calling station A transmits a FAX (or Message) signal, and, at the end of the message signal, one of EOM (End of Message), EOP (End of Procedure) and MPS (Multi-Page) signals is transmitted. Thereafter the called station B returns a message confirmation signal to the calling station A thereby completing a single facsimile communication procedure.
Although not shown in the drawings, in the case where the called station B is to function as a transmitter, when the called station B transmits a DIS signal, the calling station A returns a DTC (Digital Transmit Command) signal as a response. In succession, the called station B transmits a DCS signal and then a phase-adjustment and training signal. Then, upon returning of a CFR signal from the calling station A, a message signal is transmitted.
In accordance with the CCITT recommendations, as shown in FIG. 1b, there is a case in which the calling station A transmits a DIS signal in response to CED and DIS signals transmitted from the called station B after establishing a call between the calling station A and the called station B by dialing at the calling station A. In this case, since there is no original document to be transmitted at the calling station A, the called station B transmits a DCS signal if there is an original document to be transmitted or terminates facsimile communication if there is no original document to be transmitted.
As shown in FIG. 2, the facsimile transmission procedure recommended by CCITT (International Telegraph and Telephone Consultative Committee) includes
Phase A: Call establishment, PA1 Phase B: Pre-message procedure, PA1 Phase C1: Inmessage procedure, PA1 Phase C2: Message transmission, PA1 Phase D: Post-message procedure, and PA1 Phase E: Call return.
FIG. 3 is a flow chart showing the steps to be carried out at the receiver side in the phases B and C. It is to be noted that in FIG. 3 each step is identified by a reference numeral. As shown, after sending a CED signal, the receiver transmits a DIS signal at step 2 and determines at step 3 whether or not it has received a response from the transmitter. In the case where no response has been received, it proceeds to step 4 from where it proceeds to a tonal procedure (in this case, both of the transmitting and receiving stations are operated manually) if a GC (Group Command) signal has been received at step 4 or to step 5 if a GC signal has not been received. Then, having tried three times at step 5, it proceeds to other procedure, or after elapsing time T1 at step 6, a DCN (Disconnect) signal is sent to disconnect the transmission line.
On the other hand, if the receiver is in receipt of a response from the transmitter in response to a DIS signal, it proceeds to step 15 where the response received is checked as to whether or not it is a DTC signal. If it is, the process proceeds to D whereby the station functions as a transmitter to carry out facsimile transmission; whereas, if it is not a DTC signal, the process moves onto step 16 where the response received is checked as to whether it is a DIS signal. If it is a DIS signal, the process proceeds to E whereby an original document to be transmitted is checked and transmission is carried out; whereas, if it is not, the process proceeds to step 17 where the response received is checked as to whether it is a DCS signal. If it is not a DCS signal, the process proceeds to F to check whether it is one of EOP, EOM and MPS signals or to establish another procedure such as switching to a telephone by supplying a warning signal to the operator. If it is, in fact, a DCS signal, the process proceeds to step 18 to receive a TCF ( Training Check ) signal and then to step 19 from where, if the phase-adjustment and training check is good, the process proceeds to step 20 to transmit a CFR signal; on the other hand, if the training check is not good, the process proceeds to step 21 to transmit a FTT (Failure to Train) signal.
Then the process goes to step 11 to check whether or not the next command has been received. If negative, it proceeds to step 12 and if a message carrier is received within the delaying time period T2, it proceeds through step 14 to steps 23 and 24 where MODEM Training is carried out and then a message is received. On the other hand, if no carrier is received within the delaying time period T2, it proceeds to step 13 where the transmission line is disconnected.
If a facsimile message is to be transmitted over a long distance, e.g., from one country to another, the transmission line becomes very long so that impedance could be mismatched to produce the problem of echo interference. For this reason, an echo suppressor having the switch structure as shown in FIG. 4 is provided as connected to an ordinary transmission line. Describing the operation of the echo suppressor shown in FIG. 4, initially, both of the switches of the echo suppressor are closed to allow transmission of signals in both directions. Upon passage of the first signal, one of the switches is switched open after elapsing a predetermined delay time period to prevent the echo signal from passing therethrough. However, in the case of bidirectional data communication, the head portion of the data could be lost by the echo suppressor thereby hindering to transmit a correct signal to the other station in communication. Under the circumstances, according to the recommendations made by CCITT with respect to facsimile communication, during the call establishment step of phase A, the called station returns a CED signal, i.e., a disable tone of 2,100 Hz, to hold the echo suppressor inoperative.
In facsimile communication, especially over a long distance, echo signals are often generated. For example, as shown in FIG. 5a, when a calling station sends out a facsimile control signal comprized of a preamble PRA and a frame FRM, it will receive an echo signal of the control signal after a certain time lag, as shown in FIG. 5b. If such an echo signal has reached the calling station faster than the response signal, i.e., DCS signal in the case of FIG. 5b, from the called station, the calling station will accept the received signal at time G and therefore the calling station will receive the signal same in contents as the signal sent out by itself before receiving the DCS signal issued from the called station.
As shown in FIG. 6, the HDLC (High Level Data Link Control Procedures) frame structure is used in the binary code signal which is the standard digital facsimile signal system. In this frame structure, as shown in FIG. 6a, binary code information is always preceded by an 1 second-long preamble. Further, as shown in FIG. 6b, the binary code information is comprised of several frames such as a digital (DG) frame, a called terminal identification (RID) frame, and a non-standard facilities frame. Still further, each frame is comprised of a collection of several fields such as flag (F), address (ADR), control (CTL), facsimile control (DIS), facsimile information (FAX) and frame check (CHKF).
As shown in FIGS. 5a and 5b upon completion of sending out of a frame subsequent to a preamble at time G, the calling station accepts a receiving signal and thus when such a receiving signal is an echo signal, its frame will be processed as valid information by the calling station. Now, let us suppose that an echo signal is generated when the called station has sent out a DIS signal upon entering into phase B after sending out a CED signal for call establishment in phase A. If such an echo signal has reached the called station faster than a DCS signal issued from the calling station, then the same procedure as shown in FIG. 1b will follow, in which the calling station sends out a DIS signal in response to the DIS signal from the called station.
FIG. 7 is a time chart which contains an echo signal of the DIS signal supplied from the called station. When the called station sends out a DIS signal comprised of a preamble and a frame as shown in FIG. 7 (B1), it reaches the calling station after time lag of t1 as shown in FIG. 7 (A1). As shown in FIG. 7 (A2), after confirming carrier disconnection during time period .DELTA.t from a point H in time when the reception of the DIS signal is completed, the calling station transmits a DCS signal to the called station and the DCS signal will reach the called station after time lag of t2. It should however be noted that an echo signal of the DIS signal, which is generated at the calling station, will reach the called station after time lag of t2, as shown in FIG. 7 (B2). It usually holds that t1=t2. As is obvious from FIG. 7 (B2), since the DIS echo and DCS signals travel along the same transmission line and they are equally delayed, the DCS signal will reach the called station after the DIS echo signal.
Under the condition, if the called station accepts a receiving signal at time G when the sending of a DIS signal is completed, the called station will move into step 16 of FIG. 3 thereby regarding the frame contents of the DIS echo signal as information from the calling station. In the case of absence of an original document to be transmitted, the called station will send out a DCN signal to disconnect the transmission line; on the other hand, if an original document exists, the called station sends out a DCS signal, followed by the step of phase-adjustment and training. In any event, the calling station will incur a false operation because the transmission line is suddenly disconnected despite the fact that a DCS signal has been sent out, or training is requested after the DCS signal. An echo signal received at the called station will be accepted as a normal facsimile signal as long as it has a level in the range between 0 and 40 dbm as recommended by CCITT. Since such an echo signal is accepted and processed prior to the arrival of a normal DCS signal, the transmission line will be disconnected upon completion of communication without reception of the normal DCS signal subsequent thereto.